1. Field of the Invention
This invention relates to an optical encoding apparatus for making accurate measurements of displacement of an object with high resolution. Such an optical encoding apparatus is incorporated, for example, in a precision measuring apparatus, a drum rotation controlling device and a scanner for a copy machine, or the like.
2. Description of the Related Art
One of a conventional encoding apparatus is disclosed in German Laid-Open Patent Application (DE A1) NO. 2,316,248. FIG. 23 is an illustration showing a structure of encoding apparatus described in DE A1 2,316,248. The encoding apparatus comprises a light source 101, a lens 102 which collimates a light beam from the light source 101, two diffraction gratings 103 and 104 on which collimated light beam is incident, a condenser lens 105 and photo detectors 106, 107, 107'.
The diffraction grating 103 is fixed, and the diffraction grating 104 is movable. The pitch .LAMBDA.1 of the gratings 103 is same as the pitch .LAMBDA.2 of the grating 104. Hereinafter, the diffraction grating 103 is referred to as a fixed diffraction grating, and the diffraction grating 104 is referred to as a movable diffraction grating.
In the above-mentioned embodiment, a light beam emitted from light source 101 passes through the lens 102 and is collimated into parallel light rays. The collimated light beam is incident on the fixed diffraction grating 103 and then the movable diffraction grating 104. The collimated light beam generates at least a first diffraction beam when passing through the gratings 103 and 104. If the pitches of the gratings .LAMBDA.1 and .LAMBDA.2 are sufficiently larger than the wavelength of the collimated light beam, higher order diffraction light may be generated in each of the gratings.
FIG. 24 is an illustration explaining the diffraction light beams generated by the diffraction gratings 103 and 104. As shown in FIG. 23, a first order diffraction beam generated at the fixed diffraction grating 103 is transmitted through the movable diffraction grating 104, and received by the light receiving element 107 via the condenser lens 105. Additionally, the first order diffraction beam of the light beam transmitted through the fixed diffraction grating 103 without diffraction is generated by the movable diffraction grating 104, and is also received by the light receiving element 107 via the condenser lens 105.
As the movable diffraction grating 104 is moved in a direction indicated by an arrow R, the diffraction beams generated by the movable diffraction grating 104 are changed in their phase, while the phase of the original light beam transmitted through the fixed diffraction grating 103 and the movable diffraction grating 104 remains unchanged. That is, for example, the phase of the light beam A is not changed but the phase of the light beam B is changed. This results in phase shift of interference fringes generated by the light beams A and B on the light receiving element 107.
In this encoder, since the pitches .LAMBDA.1 and .LAMBDA.2 of two gratings 103 and 104 are equal to each other, diffraction angles of the diffraction beams having the same order at each of the gratings are the same. Accordingly, the light beams A and B are parallel to each other immediately after exiting the grating 104. If the light beams A and B are incident on the light receiving element 107 as in their parallel relationship, interference fringes generated on the light receiving element 107 have relatively large intervals. The interference fringes having such large intervals are not suitable to use for measuring the displacement of the movable diffraction grating 104 because a sufficient number of interference fringes are not formed on the light receiving element 107.
In order to form interference fringes having a suitable intervals, the condenser lens 105 is provided between the movable diffraction grating 104 and the light receiving element 107 so that the distance between the light beams A and B becomes narrow. According to this, as the movable diffraction grating 104 is displaced, the interference fringes are moved on the light receiving element 107, resulting in a sinusoidal change in the amount of light received by the light receiving element 107. Specifically, if the movable diffraction grating 104 moves a small distance corresponding to a single pitch of the grating, the level of output from the light receiving element 107 varies like a single period of sine wave. By sensing this change, the amount of the displacement of the movable diffraction grating 104 can be determined.
In the above-mentioned embodiment, although the description was given using the combination of one of the first diffraction beams generated on one side of the optical axis and the original light beam transmitted through the grating (hereinafter referred to as direct transmission beam), the combination of the other first diffraction beam may be used to form interference fringes on the light receiving element 107' as indicated by the light beams C and D in FIG. 24.
As for the light source 101 used for the above-mentioned optical encoding apparatus, a semiconductor laser (LD) is used because of requirements for compactness and high output of light. However, there is a problem in that the semiconductor laser has high dependency in its wavelength, that is, the wavelength varies due to temperature changes of the light source. Accordingly, due to the temperature change, the diffraction angle at the gratings 103 and 104 is changed, and thus the optical path in the encoding apparatus may be changed, which condition may result in that the suitable interference fringes to generate output of the light receiving element are not formed on the light receiving element. In an extreme case, the diffraction beam is directed beyond the edge of the lens 105. For example, as shown in FIG. 25, when the temperature changes, the light beams A and B may be diverted to paths indicated by the light beams A' and B', respectively. In order to avoid the effect of temperature change, the diffraction angle may be minimized by increasing the pitches A1 and A2. In such a case, however, resolution of the displacement information of the encoding apparatus may be decreased.
Another optical encoding apparatus is described in U.S. patent application Ser. No. 08/215,566, filed Apr. 18, 1994 by the present applicant. It discloses an optical encoding apparatus comprising two diffraction gratings having almost equal grating pitches, and generating interference fringes which are not influenced with a wavelength change of the light beam emitted by the light source. This disclosure is hereby incorporated by reference.
However, these types of diffraction gratings are usually difficult to manufacture because of the necessity of planning the gratings having almost equal pitches with a predetermined difference is very strict for accurate generation of interference fringes. For example, the difference in pitch which can be obtained with appropriate interference fringes for accurate measurements is within a range of 0.03% in the above-mentioned application.